Messages in this thread | | | Subject | Re: [PATCH 04/15] mm: remove the pgmap field from struct hmm_vma_walk | From | Ralph Campbell <> | Date | Fri, 16 Aug 2019 14:10:47 -0700 |
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On 8/16/19 10:28 AM, Jason Gunthorpe wrote: > On Fri, Aug 16, 2019 at 10:21:41AM -0700, Dan Williams wrote: > >>> We can do a get_dev_pagemap inside the page_walk and touch the pgmap, >>> or we can do the 'device mutex && retry' pattern and touch the pgmap >>> in the driver, under that lock. >>> >>> However in all cases the current get_dev_pagemap()'s in the page walk >>> are not necessary, and we can delete them. >> >> Yes, as long as 'struct page' instances resulting from that lookup are >> not passed outside of that lock. > > Indeed. > > Also, I was reflecting over lunch that the hmm_range_fault should only > return DEVICE_PRIVATE pages for the caller's device (see other thread > with HCH), and in this case, the caller should also be responsible to > ensure that the driver is not calling hmm_range_fault at the same time > it is deleting it's own DEVICE_PRIVATE mapping - ie by fencing its > page fault handler.
Yes, that would make it a one step process to access another device's migrated memory pages. Right now, it has to be a two step process where the caller calls hmm_range_fault, check the struct page to see if it is device private and not owned, then call hmm_range_fault again with range->pfns[i] |= range->flags[HMM_PFN_DEVICE_PRIVATE] to cause the other device to migrate the page back to system memory.
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